A fuel cell, e.g. PEFC, acts to make a hydrogen-containing fuel gas and an oxygen-containing oxidizer gas such as air electrochemically react with each other to produce electric power and heat concurrently. With regard to its structure, first, a catalytic reaction layer mainly composed of carbon powder carrying thereon a platinum-based metal catalyst is formed on both surfaces of a polyelectrolyte membrane which selectively transports hydrogen ions. Next, a gas diffusion layer having both gas permeability and electron conductivity is formed on an outer surface of the catalytic reaction layer. This diffusion layer and the catalytic reaction layer (catalytic layer) in combination serve as an electrode. A unit of this electrode and the polyelectrolyte membrane integrated together is referred to as MEA (Membrane Electrode Assembly).
Since such a gas diffusion layer is required to have gas permeability and electron conductivity, the base material therefor is given by the use of a material formed from carbon fiber such as carbon paper, carbon cloth (carbon fiber woven cloth) or carbon felt. On one side of the base material on which it contacts the catalytic layer, a conductive water-repelling layer mainly composed of fluororesin or other water-repelling resin and carbon powder is provided to serve as a gas diffusion layer, generally. The conductive water-repelling layer is effective for moisture retention of the polyelectrolyte membrane, prompt removal of any excess moisture caused by cell reaction, avoidance of any damage of the catalytic layer or the polyelectrolyte membrane that may caused by the base material, i.e., the gas-diffusion-layer base material, and other purposes.
Such a gas-diffusion-layer base material is given commonly by the use of, for example, carbon cloth (hereinafter, referred to as carbon fiber woven cloth) or carbon paper (carbon fiber nonwoven cloth). The carbon fiber woven cloth, which is a woven fabric, is formed by regularly combining together a plurality of yarns, e.g., warp and weft. Also, the carbon fiber nonwoven cloth is formed by irregular dispersion of carbon fiber.
Outside the MEA, conductive plate-shaped separators are placed for mechanical fixation of the MEA and for electrical series connection of neighboring MEAs to each other. At a portion of each separator at which it contacts the MEA, a gas flow passage is formed to serve for the supply of reactant gas to the electrode surface and for carry-off of produced gas and excess gas. The gas flow passage may also be provided independently of the separator, but it is a common method that a groove portion is provided on the surface of the separator to serve as the gas flow passage. Also, in order to prevent fuel gas or oxidizer gas supplied to the gas flow passage from leaking outside or mingling with each other, a gasket is placed around the electrode at an edge portion of the separator or along peripheries of manifolds for the fuel gas or the oxidizer gas. Thus, a unit cell for the fuel cell is made up.
A fuel-cell stack has a layer-stacked structure in which a plurality of such unit cells are stacked in layers and tightened with specified tightening power. Such tightening is intended to reduce the contact resistance between layered members, for example, at the joint portion between the separator and the gas diffusion layer, as well as to maintain the sealability for prevention of leaks of gas and circulating water.
The supply and discharge of fuel gas and oxidizer gas to and from the individual unit cells is performed through fuel-gas inlet manifold and fuel-gas outlet manifold as well as oxidizer-gas inlet manifold and oxidizer-gas outlet manifold formed on side faces of the separator. That is, the fuel gas and the oxidizer gas are supplied together through a fuel-gas and oxidizer-gas inlet formed in an end plate positioned at an end portion of the stack, and transferred dispersively to the MEAs from the inlet manifolds of the cells, respectively, through their respective gas flow passages. After the gases are subjected to electrochemical reaction at the MEAs, unreacted gas is discharged through the outlet manifolds of the cells together from a fuel-gas and oxidizer-gas outlet formed in the end plate.
As shown above, the polymer electrolyte fuel cell adopts a method that fuel gas and oxidizer gas are supplied dispersively in parallel to the individual cells from the manifolds provided at edge portions of the separators. Therefore, in the fuel cell, the gas flow rate for supply to the individual cells may differ from cell to cell if the pressure loss differs from cell to cell due to variations in the sagging amount of the gas diffusion layer into the gas flow passage that is defined by the groove portion of the separator and the gas diffusion layer, or variations in the gas permeability of the gas diffusion layer, or the like. As a result, in cells of smaller gas flow rates, there is a problem of occurrence of a voltage instability phenomenon (flooding) that water droplets block the gas flow passage, causing deficiencies of fuel supply to the electrode and the catalyst located downstream of the place of the blockage so that the voltage decreases gradually and, upon discharge of the water droplets, the blockage of the flow passage is cleared, allowing the fuel supply to recover so that the voltage increases.
As a solution to such a problem, Japanese unexamined patent publication 2003-151604 (Document 1) discloses a fuel cell, as well as a manufacturing method therefor, which is reduced in gas-flow-rate variations among the cells, the flooding thereby having been suppressed, by manufacturing the fuel cell through the steps of measuring the pressure loss for each unit cell, classifying the individual cells into predetermined ranks according to the resulting magnitudes of pressure loss, and collecting unit cells belonging to one rank.
Japanese unexamined patent publication 2004-185936 (Document 2) discloses a fuel cell in which the direction of fibers of the gas-diffusion-layer base material is set parallel to the gas flow direction so that a gas permeability of the gas diffusion layer in a direction perpendicular to the gas flow direction becomes smaller than another gas permeability of the gas diffusion layer in the gas flow direction, thus suppressing any gas leak to neighboring gas flow passages through the gas diffusion layer, with a view to reducing the pressure loss of reactant gas that flows along the gas flow passage, as an attempt at suppression of the flooding.